The present invention relates to a mixer and more particularly to a continuous mixer type that generally has a pair of rotors located adjacent to each other in parallel cylinders that form the mixer barrel.
In the manufacture of thermoplastic materials, the mixer receives the plastic material in its hopper and delivers such materials to a rotor means located in the mixer barrel wherein the rotor means advances, mixes and works the plastic materials to produce a homogeneous mix for general further processing. The art of converting solid thermoplastic materials into a satisfactory mixed and flowing homogeneous melt without overheating or chemical degradation depends in part on the type of thermoplastic material being worked on as well as the type of design of extruder or mixer used. During such mixing operation, it is essential to remove gases to obtain a high quality mix. Cost consideration requires efficient removal of gases otherwise the entrapped gases show up as porosity in the final product such as wire coating and transparent film. Entrapped gases lower the effective output capacity of the mixer by as much as twenty percent. Heretofore, degassing, if available, consisted essentially of mere venting of the mixer housing as by vent holes which became plugged with the material being mixed and advanced in the barrel. The remedy for this situation consisted of drilling the vent holes on a timed basis which could result in damage to the mixing screw or housing. The present invention contemplates the use of a vent hole or a series of holes which house a rotating vent screw. There is sufficient clearance between the vent screw and the vent hole to permit the escape of gases. In addition, the gases are permitted to escape through the screw flight openings.